A new concept in the quantification of antibiotic levels in biologic specimens has been developed utilizing enzymologic reactions to introduce specificity, sensitivity, accuracy, speed and convenience into the measurement of antibiotic concentrations. The model system chosen for initial study utilized the adenylylation of gentamicin by an enzyme derived from R factor infected bacterial cells. This model has been characterized and used in initial pharmacokinetic studies of gentamicin in children. The model was then extended to include the measurement of chloramphenicol by means of an R factor mediated enzymologic acetylation of chloramphenicol. We are currently extending these examples so that other antibiotics and chemotherapeutic agents can be measured by similar enzymologic assays.